Triggering from a unipolar pulse in the same way at different pulse amplitude values is important in many technical applications. For example, in a distance measurement technique based on the propagation time of an optical pulse, the time-of-flight of a pulse is determined based on the difference between the transmission and reception moments of the pulse. The timing of the transmission and reception moments is measured by triggering a timing comparator by means of the optical pulse.
The transmission moment is determined by reflecting a small part of the optical pulse to be transmitted, to a detector, which converts the optical pulse into an electrical pulse that is amplified in a preamplifier. For instance in constant threshold detection, the transmission moment is determined as the moment when the leading edge of an amplified electrical pulse exceeds a predetermined triggering level. Other known ways to determine the transmission moment may also be used.
The reception moment is determined in the same way. The radiation reflected from the object to be measured is received with a detector that converts the optical pulse into an electrical pulse, which is amplified in a preamplifier. In this case, too, constant threshold detection can be used to determine the reception moment by determining the moment when the leading edge of the amplified electrical pulse exceeds a predetermined triggering level. However, the problem is that the amplitude of the received pulse usually varies strongly for instance according to the distance and the reflectivity of the reflecting object. The reception moment, in turn, changes as the amplitude of the received pulse changes, i.e. a so-called walk error occurs in the triggering performed in the measurement causing even significant errors in the measurement of the time between the transmitted pulse and the received pulse. A walk error occurs because of the unipolar optical pulse, and, characteristically, the unipolar pulse has no such feature that would enable triggering and timing without walk error.
The CF (Constant Fraction) principle has been used when trying to alleviate the problem, and, at its simplest, can be implemented using a high-pass circuit at the input of the timing comparator. The high-pass circuit converts the unipolar pulse into a bipolar pulse, whose zero crossing point is expressed with the timing comparator. A timing comparator operating according to the CF principle is able to trigger the timing exactly from a pulse in a given dynamic range whose width depends on the acceptable timing error. However, converting a pulse to bipolar does not succeed when the amplitude of an electrical pulse is cut in amplifiers preceding the timing comparator. This is why the CF principle is easily applicable only with automatic gain control (AGC), which keeps the amplitude of the pulse incoming to the timing comparator within a reliable operational range of the timing comparator operating according to the CF principle. However, the problem associated with triggering is not solved this way, but a new problem is created, i.e. that the delay caused to the electrical pulse by the AGC circuits varies according to the amplification of the AGC circuit, i.e. the dynamics are limited in a given measurement error range. The technology is also complicated. In addition, several extra measurement pulses have to be transmitted to the object to determine the amplitude information required as the basis of the control.
A simpler method for timing detection is to operate without automatic gain control. When a timing comparator that detects the edge of a unipolar pulse is used, the amplification of the amplifier succeeding the detector is kept constant irrespective of the amplitude of the input signal. This causes problems. The amplification must be adjusted such that even a small input signal is of sufficient magnitude when applied to the detection block of the timing point. Because of the constant amplification, stronger input pulses grow so large that they are clipped in the amplifiers. Furthermore, the magnitude of the timing error depends on the rate of rise of the pulse, etc. The timing error reaches its peak when one or more amplifiers are saturated. In all, the problem in edge detection is insufficient accuracy.